A disk drive system includes one or more magnetic recording disks and control mechanisms for storing data on the disks. The disks are constructed of a substrate and multiple film layers. In most systems, an aluminum-based substrate is used. However, alternative substrate materials such as glass have various performance benefits such that it may be desirable to use a glass substrate. One of the film layers on a disk is a magnetic layer used to store data. The reading and writing of data is accomplished by flying a read-write head over the disk to alter the properties of the disk's magnetic layer. The read-write head is typically a part of or affixed to a larger body that flies over the disk, referred to as a slider.
The trend in the design of magnetic hard disk drives is to increase the recording density of a disk drive system. Recording density is a measure of the amount of data that may be stored in a given area of a disk. For example, to increase recording density, head technology has migrated from ferrite heads to film heads and later to magneto-resistive (MR) heads and giant magneto-resistive (GMR) heads.
Current disk drive products use longitudinal magnetic recording technology. However, perpendicular magnetic recording systems have been developed to achieve higher recording density. A typical perpendicular recording head includes a trailing write pole, a leading return or opposing pole magnetically coupled to the write pole, and an electrically conductive magnetizing coil surrounding the yoke of the write pole. The bottom of the opposing pole has a surface area greatly exceeding the surface area of the tip of the write pole. Conventional perpendicular recording media typically includes a hard magnetic recording layer and a soft magnetic underlayer which provide a flux path from the trailing write pole to the leading opposing pole of the writer. To write to the magnetic recording media, the recording head is separated from the magnetic recording media by a distance known as the flying height. The magnetic recording media is moved past the recording head so that the recording head follows the tracks of the magnetic recording media, with the magnetic recording media first passing under the opposing pole and then passing under the write pole. Current is passed through the coil to create magnetic flux within the write pole. The magnetic flux passes from the write pole tip, through the hard magnetic recording track, into the soft underlayer, and across to the opposing pole.
Achieving higher areal density (i.e., the number of stored bits per unit surface area) requires that the data tracks be close to each other. One problem with current perpendicular magnetic recording media is that because the soft magnetic underlayer contains magnetic granular structures that are exchange coupled in the plane of the substrate, a large number of magnetic domains within the soft magnetic underlayer are formed. As such, any magnetization transition in the soft magnetic underlayer would be at least as broad as a typical domain wall width, thereby limiting how narrow a data track may be. This is a problem because sharp head field gradients are needed to write narrow transitions in the perpendicular hard magnetic films.